Direct steam injection is e.g. used in the food industry for pasteurising or sterilising of milk or cereal products. Various types of apparatus for pasteurising or sterilising fluid food products by injection of steam are known. The present invention relates to a method and apparatus wherein steam is injected into a product stream.
Whereas direct steam injection does not raise particular problems for low viscosity fluids such as milk, whey and cream, the pasteurisation of e.g. infant cereal based food, with far higher viscosity, e.g. comprising 20% starch, is very difficult to control in a homogeneous mixing. An inhomogeneous mixing will mean that temperature varies through the product mass and consequently the product is not homogeneously heat treated. In order to safeguard sufficiently heat treatment of the product i.e. to make sure that all parts of the product mass are heated to a high enough temperature, such as is crucial in pasteurising and sterilising processes, additional steam will need to be injected. This additional steam consumption is expensive and undesirable.
Homogenisation and mixing may be improved by passing the product through a venturi shaped nozzle in order to increase turbulence and velocity of the product, however, this may not provide a sufficient enough mixing. Alternatively, a mechanical mixing of the product and the steam may be necessary.
Mixing of the product and steam may be obtained by passing the product through a static mixer. However, substantial wear of the static mixer is likely to result from the passage of highly viscous products there through. Such static mixer will therefore have to frequently be replaced, which increases production costs. Alternatively, a screw may be used for mixing and for building up a pressurised product. However, the product creates a large momentum on the screw when rotated, and hence considerable motor capacity is required for the mixing.